Biased crossed field dynamic electron multiplier



June 11;, 1968 R, B. HANK|N ETAL 3,388,282

BIASED CROSSED FIELD DYNAMIC ELECTRON MULTIPLIER Filed March 29, 1965 ggg-1 United States Patent O 3,388,282 BIASED yCRGSSEI) FIELD DYNAMIC ELECTRGN MULTIPLIER Robert B. Hankin, Ernest P. Dallafior, and Bernard P.

Alpiner, Chicago, Ill., assignors, by mesne assignments, to The Hallicrafters Co., a corporation of California Filed Mar. 29, 1965, Ser. No. 443,222 7 Claims. (Cl. 315-4) ABSTRACT OF THE DISCLOSURE A crossed-field dynamic electron multiplier in which the two plates which define the path of electron travel are insulated from each other and an electrostatic bias is maintained between them with the emissive plate at a negative potential with respect to the other plate.

This invention is concerned with an improved multiplier tube and more particularly with an improved microwave dynamic multiplier tube.

Such a tube has been proposed by Gaddy and Holshouser in an article entitled, A Microwave Frequency Dynamic Crossed-Field Photomultiplier, Proceedings of the IEEE, vol. 51, pp. 153-162, January 1963. The tube described in such article has a pair of spaced plates which define a path of electron travel and are located in a cavity which is excited at microwave frequency. The electrons are generated, as by a photo cathode, at one end of the spaced plates and are propagated down the path between them to a collector at the opposite end. A fixed magne-tic field extends across the path of electron travel with the magnetic lines of force at right angles to the path and generally parallel to the plates. The surface of one of the plates is particularly adapted to emit secondary electrons when primary electrons impinge on it. The electrons travel between the plates with an increase in the number of electrons, or current amplification.

A serious problem with the prior tube is its instability. It is relatively difficult to adjust the various operating conditions of the prior device for satisfactory operation and without encountering an unstable condition known as multipactoring Proper operation requires that a relatively delica-te balance be maintained between the magnetic and electric fields. Furthermore, the known multiplier tubes require a high level of radio frequency exci-tation, resulting in low over-all etiiciency for the stage.

One feature of the present invention is the provision of such multiplier tube in which means are provided for establishing an electrostatic field at right angles to -the path of electron travel and to the magnetic field. More particularly, one of the two plates which defines the path of electron travel is provided with an insulated mounting and an electrostatic field is established between the two plates. This field has the principal advantage of reducing instability in the tube, and has the added advantages of reducing the required RF power input and providing an increase inthe output rating of the tube.

Another feature of the invention is the provision of 3,388,282 Patented June l1, 1968 ICC such a tube in which the secondary emissive plate has a high impedance with respect to the cavi-ty wall for direct current and a low impedance to the cavity wall at the excitation frequency. This eliminates spurious effects of the high frequency excitation between the plate and its port.

Further features and advantages of the invention will readily be apparent following the specification and from the drawings, in which:

FIGURE 1 is a longitudinal section through a tube embodying the invention;

FIGURE 2 is a transverse section taken generally along line 2 2 of FIGURE 1; and

FIGURE 3 is a transverse section, similar to that of line 2 2 of FIGURE 1; and

FIGURE 3 is a transverse section, similar to that of FIGURE 2, illustrating a modified tube construction.

The 4tubes suggested by Gaddy and Holshouser and disclosed herein are particularly suited for the detection of light beams modulated at a high frequency, as for example in the gigicycle range, and the amplification of the resulting electric-al signal. For example, such tube can be used as a detector-amplifier in a Laser communication system or in a microwave modulated optical radar. Other uses will be apparent to those skilled in the art.

Turning now to the drawings :and more particularly to FIGURES 1 and 2, the tube shown therein has an evacuated enclosure 19, here a sealed housing with a conductive inner surface 1=1, which may suitably be formed of a metal material. A path for the propagation of electrons is defined by a pair of parallel plates 12 and 13, there being an electron source 14 at one end of the plate 13. Plate 12 is mounted on a conductive post 12a extending from the cavity wall and is at the same direct potential as the housing. The electron source in this instance is a photo sensitive cathode which is subject to external light directed through lens 15 and a suitable aperture 16 in the cavity wall. Electrons emitted from photo cathode 14 provide an electric signal which represents the modulations of the light beam.

Plate 13 has a surface 13a which is particularly suited for the emission of secondary electrons when electrons impinge upon it.

A magnetic field extends at right angles to the path of electron travel and, in the embodiment of FIGURE 1, has a sense in which the lines of flux appear to come out from the surface of the paper. The magnetic field may, for example, be established by a suitable electromagnet having poles 19 and 20 located on either side of the tube and extending the length of the electron path, FIGURE 2.

The interior of housing 10 has dimensions such that it has a resonant condition at a suitable frequency, as in the microwave range. The interior of the cavity is excited at its resonant frequency by energy from a suitable source .(not shown) injected at 21. The electrons emitted from photo cathode 14 are accelerated between the two plates 12 and 13 by the radio frequency field. The presence of the magnetic eld causes these electrons to traverse a curved path and, upon reversal of the radio frequency field, the electrons are driven back toward the plate 13, against which a substantial number of them impinge. Electrons ltasaasz striking this surface cause emission of secondary electrons which follow a similar trajectory along the electron path between the plates. At the end of plate 13 opposite photo cathode 14 is a collector 22 connected wilh a suitable load 23 which is returned to a reference potential or ground 24. Further information regarding the nature of ythe operation of the tube may be found in the Gaddy and Holshouser publication identified above.

The tube disclosed `herein differs from that of Gaddy and Holshouser in that an electrostatic field is established `between plates 12 and 13. In order to establish this electrostatic field the plate 13 is electrically insulated from the inner surface 11 of the cavity for direct current. ln the embodiment of FIGURES l and 2, the plate 13 is spaced from the cavity wall by a suitable insulating sheet 26, as of mica or the like. A negative potential is applied to plate 13 as 'by connecting it with the negative terminal of a direct current power supply, the positive terminal of which is connected with the housing 10 and collector 22, through a common ground circuit. The electrostatic field resulting from the direct current potential applied to plate 13 extends between plates 12 and 13, generally at right angles to the path of propagation of electrons between the plates and at right angles to the magnetic field. Experiments have shown that the electrostatic field has la principal advantage of reducing the instability in such a tube, believed to be due to a phase instability or "multipactoring effect in which electrons would follow the repetitive trajectory 30 between the two plates, without contributing 'to the total output current of the device. The addition of the electrostatic field reduces the tendency of the electrons to take up a multipactoring trajectory. In addition to improving the stability of the tube, the current output is increased for a tube of given physical dimensions and materials and the level of the radio frequency input power required for optimum operation is reduced.

In a representative example of tube utilizing such an electrostatic field, the plates 12 and 13 are spaced apart a distance of three millimeters. The DC voltage applied to the plate 13 and thus between the two plates is of the order of several hundred volts, as, for example, 300 Volts. The length of the electron path from photo cathode 14 to collector 22 is 28 millimeters. The secondary electron emitting surface of plate 13 is a beryllium-copper rent output of the tube is increased by the addition of the p electrostatic field. It is believed this may be due to the repulsive effect Ibetween the negatively charged plate 13 and the electrons by virtue of which the electrons are caused to travel from the plate -to the grounded collector 22. Without the direct voltage relationship, the high level of output current cannot be achieved without providing some more complicated bias arrangement.

The optimum strength of the electrostatic field is related to the strength of the radio frequency field induced between the two plates 12 and 13 by the radio frequency power injected at 21. In general, the output of the tube increases .as the direct voltage is increased. However, when the direct voltage is increased above the RMS value of the alternating field induced between the two plates, the increase in the output falls off.

It sometimes happens that interference is caused by alternating high frequency energy setting up spurious currents between the plate 13 and the adjacent wall of housing 10. This can occur at the frequencies used in this type of device even though the space is filled with insulating material, as mica. For example, with .an openating frequency of 3200 megacycles, the spacing between the two surfaces, even though small, may ybe .an appreciable portion of a wave length. The modified construction illustrated in FIGURE 3 shows a solution for this problem. Here, plates `30 and 31 define the path of electron travel within cylindrical housing 32. In this case, the path `of travel is at right angles to the plane of the paper. Plate 31 is insulated from housing 32 for direct current, as by dielectric spacer 33. The lower surface 31a of plate 31 conforms generally with the circular configuration of the housing but has an effective outer diameter slightly less than the inner diameter thereof. Plate 31 has a cross-sectional extent somewhat less than one-half the cross-sectional area of the housing.

To prevent spurious currents between plate 31 and the inner wall of Vtube 32,`it is desirable that the impedance, `at the excitation frequency, between the edges 34 of plate 31 and the adjacent walls of the housing should be extremely low. In the embodiment of the invention illustrated this is achieved by the physical configuration of the plate. The distance between plate edges 34 and the central mounting portion 31b of the plate is one-quarter wave length at the excitation frequency. This is effectively an open quarter wave length stub which appears -at the plate edge 34 .as a short circuit. This condition is enhanced by the provision of slots 35 in the under surface of plate 31 which form a shorted quarter wave length section in series with each of the open quarter wave 4length sections .between the plate and the cavity wall.

While We have shown and described certain embodiments of our invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit .and scope lof the invention as disclosed in the appended claims.

We claim:

1. In a crossed-field multiplier tube having a conductive resonant housing, a source of electrons, a pair of `spaced plates defining a path of electron travel along which electrons from said source are propagated, one of said plates having an electron emissive surface, means establishing a magnetic field at right angles to said path, and means for exciting said housing at its resonant frequency, the improvement comprising: means insulating said plates from one another for direct current; and neans applying a direct current bias between said plates with said one `plate being negative with respect to the other.

2. The crossed-field multiplier tube of claim 1 wherein said plates are mounted on said housing and said one plate is separated therefrom by an insulating member.

`3. The multiplier tube of claim 1 in which the strength of the electrostatic field between Vsaid two plates is of the same order as the RMS value of the alternating field induced between the two plates by excitation of the housing.

4. In a crossed-field multiplier tube having a conductive resonant housing, a source of electrons, a pair of spaced plates defining a path of electron travel along which electrons from said source are propagated, one of said plates having an electron emissive surface, means establishing a magnetic field at right angles to said path, and means for exciting said housing at its resonant frequency, the improvement comprising: means insulating said plates from one another for direct current; and means insulating said one plate and electron emissive surface from the conductive surface of said housing for direct current, said one plate having an edge surface extending from said emissive surface generally parallel with and spaced from said housing wall for a distance substantially equal to an odd multiple of one quarter wave length at the excitation frequency establishing an open quarter wave length scction extending from said edge, and a low impedance at said radio frequency between the edge of said plate and said housing surface.

5. The multiplier tube of claim 4 wherein said electron emissive plate has a slot therein detining a shorted quarter wave length section in series with said open quarter wave length section.

6. The multiplier tube of claim 5 wherein said plate has opposed edges spaced between opposed surfaces of said cavity, with series connected quarter wave open and shorted sections extending from each of said edges to present a low impedance to said radio frequency excitation between each edge and the adjacent cavity surface.

7. The multiplier tube of claim 6 including a source of direct current connected with said plate and establishing an electrostatic field at right angles to said path of electron travel and said magnetic eld.

References Cited UNITED ll/l939 12/1949 8/1950 4/1954 1/1957 l/l966 2/1966 STATES PATENTS Farnsworth et a1. S15-39.3 X

HERMAN KARL SAALBACH, Prmm'y Examiner.

S. CHATMON, IR., Assistant Examiner. 

